Pure titanium exists in the alpha crystalline form at room temperature, but transforms to the beta crystalline form at 1621.degree. F. (883.degree. C.). Various alloying elements increase the stability of the beta phase at lower temperatures. Certain known titanium alloys contain sufficient amounts of the beta phase stabilizers that they are largely beta phase under most temperature conditions and are referred to as beta titanium alloys. The subject of these prior "beta" titanium alloys is discussed in "The Beta Titanium Alloys," by F. H. Froes et al., Journal of Metals, 1985, pp. 28,37.
Titanium alloys possess an ideal combination of strength and low density for many aerospace applications, including gas turbine engines, and particularly gas turbine engine compressor blades, vanes and related hardware. However, titanium is a highly reactive metal and can undergo sustained combustion under conditions encountered in gas turbine engine compressors. In such compressors, ambient air is compressed at temperatures on the order of 850.degree. F. (454.degree. C.) to pressures which may be on the order of 400 psi. The air can flow at 450 feet per second as it passes through the compressor. Under these conditions common commercial titanium alloys will burn uncontrollably if ignited. Ignition can occur by friction arising from the ingestion of foreign objects or as a result of mechanical failures which cause contact between moving blades and stationary objects, at least one of which is made of titanium alloy, with friction between two titanium components being particularly troublesome. Such combustion is a great concern to gas turbine engine designers who have gone to great lengths to guard against rubbing between titanium components.
A class of true beta titanium alloys based on compositions of titanium-vanadium-chromium which occur in the titanium-vanadium-chromium phase diagram bounded by the points Ti-22V-36Cr, Ti-40V13Cr and Ti-22V-13Cr (all percentages herein being weight percent unless otherwise noted) has been shown to possess a high degree of resistance to burning (referred to hereinafter as non-burning) under the operating conditions in a gas turbine engine. These alloys also exhibit creep strengths which are greater than those exhibited by the strongest commercial alloys (i.e., Ti-6-2-4-2) at elevated temperatures. A variety of quaternary (and higher) alloying elements may be added to the basic composition to modify the alloy properties.
A particular titanium base alloy, having a nominal composition of 35% V, 15% Cr, balance Ti, has been historically used for gas turbine applications in the fully solutioned (all beta) condition. When operating above 850.degree. F. (454.degree. C.) for extended periods of time, alpha phase precipitates as an essentially continuous film in the grain boundaries and embrittles the alloy, thus shortening its useful lifetime.
What is needed is a non-burning titanium alloy which can operate for extended periods of time at elevated temperatures without becoming embrittled.
What is further needed is a method of heat treating a non-burning titanium alloy so as to render it resistant to the embrittling effects of long term exposure at elevated temperatures.